Method of forming a trench gate MOSFET having a thick bottom oxide

A method of forming a trench gate MOSFET comprising:

• forming an epitaxial layer with a first conductivity type on a substrate with the first conductivity type;
• forming a trench in the epitaxial layer;
• conformally forming a first insulating layer on surfaces of the epitaxial layer and the trench;
• forming a first conductive layer at a bottom of the trench;
• removing a portion of the first insulating layer to form a second insulating layer exposing an upper portion of the first conductive layer;
• performing an oxidation process to oxidize the first conductive layer to a third insulating layer, wherein a fourth insulating layer is simultaneously formed on the surface of the epitaxial layer and on a sidewall of the trench by the oxidation process;
• forming a second conductive layer in the trench;
• forming two body layers with a second conductivity type in the epitaxial layer respectively beside the trench;
• and
• forming two doped regions with the first conductivity type in the body layers respectively beside the trench.